1. Field of the Invention
The invention is directed to an improved fuel injection system for internal combustion engines, including a pressure booster between the fuel injector and a high pressure source.
2. Description of the Prior Art
From German Patent Disclosure DE 43 11 627, fuel injection systems are already known in which an integrated pressure booster piston, by means of filling or evacuating a return chamber, makes it possible to increase the fuel injection pressure above the value furnished by a common rail system.
The fuel injection system of the invention has the advantage over the prior art that because triggering is done exclusively via the return chamber of the pressure booster, the triggering losses in the high-pressure fuel system, by comparison with triggering via a work chamber communicating intermittently with the high-pressure fuel source, are less. Moreover, the high-pressure region, and in particular the high-pressure chamber, is relieved only down to rail pressure and not down to the leakage level, which improves the hydraulic efficiency.
By using a fast-switching piezoelectric valve as the control valve, small injection quantities can be injected into the combustion chamber of an internal combustion engine in a defined way and with small variations in quantity even when the nozzle opening pressure is high; moreover, because of the fast switching, only slight leakage losses occur.
A disposition of the pressure booster coaxially to the closing piston advantageously makes a small-volume, economical design possible.
A variation in the switching speed especially in a piezoelectric valve that has an essentially linearly triggerable piezoelectric actuator makes it possible to change the pressure increase gradient at the onset of injection, or in other words to shape the course of injection, and thus enables optimal adaptation of the course of injection to the requirements of the engine.
By using a fast-switching piezoelectric valve as the control valve, small injection quantities can be injected into the combustion chamber of an internal combustion engine in a defined way and with small variations in quantity even when the nozzle opening pressure is high; because of the fast switching, only slight leakage losses moreover occur.
A variation in the switching speed especially in a piezoelectric valve that has an essentially linearly triggerable piezoelectric actuator makes it possible to change the pressure increase gradient at the onset of injection, or in other words to shape the course of injection, and thus enables optimal adaptation of the course of injection to the requirements of the engine.
If a 3/3-port directional piezoelectric activated valve is used, then the intermediate position can be realized by a partial stroke of the piezoelectric actuator and used to create an injection at low pressure. This also makes a shaping of the course of injection possible and in particular a boot injection and improves the metering of small fuel quantities.
A further-improved needle closure is achieved by an optimized hydraulic adaptation, in particular of a filling path of the high-pressure chamber. To that end, an acceleration phase is generated, in which the pressure in the nozzle chamber is less than the pressure in the needle pressure chamber. The result is an additional hydraulic closing force on the nozzle needle, and the acceleration phase upon closure can be shortened sharply. Because of the faster needle closure, the characteristic quantity curves in ballistic operation become shallower. As a result of this hydraulic supplementary force, very stable needle closure and thus a very stable end of injection are achieved. This increases the metering accuracy of the injector. Moreover, a faster reaction of the nozzle needle to the control signal end is achieved, as a result of which a shallower characteristic quantity curve in the ballistic range is achieved, and the metering accuracy is enhanced still further. Simultaneously, because of the faster needle closure, an improvement in engine emissions can be expected.